To add those strings, herez da codez:
listbox.Items.Add("DTR ON TIME");
listbox.Items.Add("DTR OFF TIME");
Seriously though, are you asking for the time at which DTR was turned on/off?
If so, that would go something like this:
listbox.Items.Add(string.Format("{0} - DTR OFF", DateTime.Now.ToString("yyyyMMdd HH:mm:ss")));
listbox.Items.Add(string.Format("{0} - DTR ON", DateTime.Now.ToString("yyyyMMdd HH:mm:ss")));
Of course, you'd probably have to handle a custom event and invoke the proper delegate in order to add the appropriate text to the listbox, but you didn't ask for code for that, so I assume you know what you're doing.